Nuclear power plants remain a major source of clean energy. For all its benefits, the awesomeness of nuclear energy evokes (justified) fear of uncontrolled heat, devastating human loss, and lingering radioactive contamination. We remember places like Three Mile Island and Chernobyl, and more recently, Fukushima, from the “devastation,” “destruction,” “meltdown,” “Armageddon” and “catastrophe” that happened and left their mark there.
Calling these tragedies “accidents” does not trivialize the loss but rather, underscores the potential for human error and the need for redundant containment systems that take this into account. Uranium is the most abundant and best-known source of nuclear power. Uranium dioxide is compacted into small cylinders, which are in turn stacked to form longer cylindrical fuel rods. Fuel rods are clad in neutron-absorbing steel or zirconium alloy. Fuel rods are bundled together to form a fuel assembly. Uranium naturally decays, producing neutrons, which in turn provoke other nearby uranium atoms to decay. This is the chain reaction. This reaction generates extreme amounts of heat quickly. To moderate this reaction, and contain the heat, the fuel assembly is bathed in cooled water and retained within a reinforced pressure vessel. In this way, the pressure vessel is like a self-heating tea kettle, but with two spouts. It has an inlet for cool water, and a corresponding outlet for boiling water or steam. Downstream of the pressure vessel, the outlet carries the resulting steam to a turbine. The force of the steam rotates the turbine to generate electricity, which can be delivered to homes and businesses. A pump further downstream from the turbine sends the steam to be cooled and condensed into liquid water. This water is recirculated to the “kettle” to continue cooling the fuel assembly.
As long as there is enough volume of cool water to cover the fuel, the system can keep the pressure vessel to about 500 F. If the pump fails, not enough cool water fills can cover the fuel assembly. The fuel assembly continues to generate heat from fission. While the ambient air does dissipate some of the heat, it does not do so as well as water. Heat builds, as it is not removed by the normal circulation of water through the pressure vessel. Control rods dipped into the water bath moderate and stop the chain reaction itself but are not enough to remove the heat of the reduced, but ongoing, fission. Unchecked, the temperature in the pressure vessel can rise to 2200 F. This extreme heat must go somewhere, so, after the remaining water is boiled off, it bursts or melts the pressure vessel, and possibly the containment structure as well.
The pressure vessel is typically made of reinforced steel or other strong material to withstand the pressure of the chain reaction and also absorb neutrons. The pressure vessel is not indestructible. Given enough heat, the steel will melt, forming a pool on the containment vessel floor. Given enough pressure, the pressure vessel will rupture. Therefore, there must be a way to remove the heat and contain the radiation in case tie pressure vessel fails. Current practice is to contain the pressure vessel within some kind of containment structure. Containment structures are often made of reinforced concrete or steel, and do just that, provide backup heat and radiation containment means.